In the prior art a common type of electrical coil is the layer-wound solenoid coil, illustrated in FIG. 1. Typically, this type of coil is fabricated by layer-winding electrical conductor 10 onto the outside of a winding cylinder 12 much the same as sewing thread or yarn is layer-wound onto a bobbin. Coil winding is usually done by mechanically rotating the winding cylinder 12 and guiding the conductor 10 onto the surface of the cylinder with the conductor 10 advancing one conductor width per revolution. When the surface of the winding cylinder 12 is covered by conductor 10, the first layer 14 is complete and the second layer 16 is wound on top of the first layer 14. The winding cylinder is rotated in the same direction for the second layer 16, but the conductor 10 advances in the opposite direction so that the second layer 16 ends at the same end of the coil as where the first layer started. Coil winding continues in this manner by winding from end-to-end and progressing from layer-to-layer with the radius of each new coil layer one conductor thickness larger than the last layer.
One of the conditions for constructing a good electrical coil is that the current must flow along the electrical conductor 10. This requirement is met by using electrical insulation between conductors to prevent turn-to-turn and layer-to-layer shorts. The electrical insulation is usually applied to the conductor before winding, but in some cases the electrical insulation is done in two stages. The first stage is to insulate the surface of the conductor so that the bare metal surfaces of adjacent conductors do not contact during the coil-winding process. For this example, the first stage is accomplished by wrapping the bare conductor with high-temperature glass tape 18. The porous glass tape 18 serves as a spacer between coil turns during winding and it becomes a reinforcement to the epoxy in the second stage. The second stage of electrical insulation is to vacuum impregnate the coil winding with a thermoset epoxy The epoxy impregnation fills all of the voids in the coil winding, which includes the voids in the glass tape 18. The thermoset is then cured to bond the coil winding into a monolithic coil structure.
The coil construction shown in FIG. 1 has been used to build conventional water-cooled electromagnets for many years. These methods work well for low-current-density/low-field coils, but they are lacking in adequate structure for high-current-density/high-field coils. The following discussion describes the Lorentz forces that are generated inside the windings of a solenoid magnet and considers the trade-off choices that can be used to design a good reinforcement structure to react the Lorentz forces.
When a current flows through an electrical coil, a magnetic field is generated and the current-carrying coil conductors are affected by this magnetic field. The current-carrying conductors experience electromagnetic forces due to the magnetic field, and these electromagnetic forces are called Lorentz forces. For solenoid coils, the dominant Lorentz forces are directed radially outward, and they are applied to the coil conductor. Axial Lorentz forces which are directed inward are also applied to coil conductors near the ends of the solenoid coil, but effects of these forces are easier to negate with coil structure. This discussion is a treatment of the more difficult radial Lorentz forces only.
The electrical insulation used between adjacent coil turns is not a good structural material. The insulation is often reinforced with glass or similar fibers, but the insulation-to-conductor epoxy bond is left as the weak link in the structure. The lack of confidence in the conductor-to-insulation bond in superconducting magnets is particularly worrisome. Superconducting magnets are cooled to 4.degree.-5.degree. K. for operation, and insulation materials tend to become brittle at these low temperatures. Furthermore, insulations typically shrink more than conductor materials, and the cooldown differential contraction tends to load the insulation-to-conductor epoxy bonds into tension. Also, for thick solenoids with outside-to-inside radius ratios greater than 1.85, the radial stress distribution due to Lorentz forces changes from compression to tension and the conductor-to-insulation bond in the radial direction is loaded in tension. This tension loading in the insulation-to-conductor bond is not acceptable for magnets that must work reliably. A failure of this bond can lead to insulation damage, followed by layer-to-layer shorts. This lack of confidence in the conductor to-insulation bonds to carry tension has led to the adoption of an engineering design requirement for some superconducting magnet applications of no tension allowed in the insulation-to-conductor epoxy bonds.
Insulation-to-conductor epoxy-bond tensile stresses may be eliminated by preloading. If the bond is preloaded into compression, tension excursions that follow the preloading will reduce the compression loading. If the coil is preloaded into compression and the conductor-to-insulation compression preloading is greater than the tension loading due to cooldown or Lorentz forces, the conductor-to-insulation-bond stresses will remain in compression. Therefore, the coil must be preloaded into compression to an amount greater than the magnitude of the tension excursions that follow if insulation-to-conductor epoxy-bond tensile stresses are to be eliminated.
One method of accomplishing preloading in the radial direction is to wind the coil with tension in the conductor. The conductor is stretched as it is being wound onto the coil, and this stretching develops radial compression between coil layers when the conductor is in place. This technique has also been used to fabricate cylindrical pressure vessels. If the outside of a cylinder is wrapped with material which is stretched during winding, the bore of the cylinder is preloaded into compression and the allowable operating pressure of the vessel is higher than a solid cylinder of the same thickness.
A second method of accomplishing preloading in the radial direction is to shrink a cylindrical jacket onto the outside of the coil. This technique works well for conventional coils which operate near room temperature, and it has also been used to shrink the field-shaping iron onto the outside of superconducting coils. This shrink-fit method has also been used for many years to build high-pressure vessels and gun barrels. The inside cylinder or coil is cooled to decrease its outside diameter or the outside cylinder is heated to increase its inside diameter. In some cases, both heating of the outside jacket and cooling of the inside cylinder is used to accomplish a maximum interference between cylinders. When both cylinders are in the ready-to-assemble condition, the outside diameter of the inner cylinder is smaller than the inside diameter of the outer cylinder. The cylinders are then assembled together and allowed to warm up to room temperature. At room temperature, both cylinders are preloaded into compression in the radial direction.
A third method of accomplishing preloading in the radial direction is similar to the shrink-jacket method described above. This differential contraction of materials method works well for devices that operate at a temperature which is different than the fabrication temperature. The shrink-fit method uses expansion and contraction due to temperature differences to develop an interference fit during the assembly process to develop radial preloading. The differential contraction of materials method uses different materials with different expansion coefficients to accomplish radial preloading. This method works well for superconducting coils in that assembly takes place at room temperature, and the operating temperature is 4.degree.-5.degree. K. This temperature excursion, referred to as cooldown, has been used to accomplish radial preloading of superconducting coils.
The three methods described above to accomplish preloading in the radial direction may also be used in combination. If the operating temperature is different than the fabrication temperature, the preload methods used for fabrication may be combined with the differential contraction of materials to maximize the preloading in the radial direction.
Some superconducting materials are brittle. To form these materials into a coil, the materials are formed into a ductile nonsuperconducting wire. The wire is wound into a coil, and then the coil is heated causing a reaction making the wire superconducting and brittle.
There is a need to provide a higher compressive preloading in the radial direction for superconducting coils which use brittle superconducting materials which must be reacted after winding and which are used in a coil to produce high magnetic fields.